The variable-quantity fertilization technology is a core of the precision agriculture system. An intelligent variable-quantity fertilization operating system rapidly obtains information such as soil and crop growth statuses, and uses a crop nitrogen diagnosis model as a decision to change application of fertilizer in real time, so as to improve fertilizer utilization efficiency, reduce waste, protect the environment, and increase incomes of farmers. Obtaining crop growth information online is a primary condition for performing variable-quantity fertilization operations. Currently, crop growth spectrum monitoring mechanisms in China and abroad are all remote sensing methods of vertical angulation by using a sensitive element of a crop growth multispectral sensor. To be specific, the sensor is vertical to a crop canopy and performs static testing at a specific height. In agricultural machinery variable-quantity fertilization operations, because farmland roads are uneven, when traveling in the fields, agricultural machinery may tilt or bump. Consequently, the sensitive element of the crop growth multispectral sensor cannot perform vertical testing, directly affecting precision of crop canopy reflectance spectrum measurement. In addition, because of mechanical vibrations of the agricultural machinery, and because a sensitive element cantilever of the crop growth multispectral sensor is a weakly damped elastic body, when the agricultural machinery vibrates, the sensor vibrates correspondingly, resulting in that the sensor cannot obtain canopy reflectance spectrum information statically. Consequently, the measurement is accurate.